1. Field of the Invention
This disclosure sets forth pixel elements which uses polymer electroluminescent materials, and more particularly integrally fabricated pixel elements and control circuitry for flexible, flat panel displays.
2. Background of the Art
Early prior art electroluminescent (EL) devices used in flat panel displays are called are diodes in which applied alternating current (AC) or direct current (DC) or pulse potentials affect luminescence. The diode or two terminal embodiment of flat display panel pixels presents significant operational and manufacturing limitations. One limitation is in the form of a barrier contacts at both the injector and the collector terminals of the diode. These barrier contacts increase significantly the potential required for the luminescence, and decrease the operational lifetime of the device because of cumulative terminal EL material interface stress. The stress of the high field of the non-ohmic contacts to the EL material affects the interface therebetween, degrading operation and causing failure. Another limitation is the increased complexity of address and intensity modulation necessary for use as pixel elements in information display.
Still another limitation of prior art EL devices is that the address and intensity modulation circuitry must be separately manufactured and assembled to prior art diode pixel display devices thereby increasing the cost of the display product. Another limitation is the power requirements for the control circuitry which are orders of magnitude greater than the control circuitry for the presently disclosed device. Still another limitation of the diode pixel element is that light is emitted through the diode's transparent contact and not laterally as will be expanded upon in the following paragraph. This results in a significant percentage of light emission that is not utilized, thereby further increasing the power required in the prior art to obtain the desired level of luminescence.
In the prior art DC diode devices, one contact to the EL material of the diode is made by transparent indium-tin oxide (ITO), and the other by a metal which is typically Al. Both of those contacts are Schottky barrier, tunneling contacts. A reverse bias applied to the EL material produces a field across the depletion region. A sufficient field causes avalanche of energetic carriers which are typically electrons. The electrons impact excite centers, or color centers of the EL material, creating electron-hole pairs, and/or excitation of the color dopant atoms. Relaxation of the excitation within the EL materials causes photon, colored light, emission. Only the photons exciting the EL material parallel to the field produced laterally, perpendicular to the field, is essentially lost and does not markedly contribute to the brightness of the viewed light of the prior art.
U.S. Pat. No. 5,656,883 Alton O. Christensen, Sr., which is assigned to the assignee of the present disclosure and which is entered herein by reference, discloses a triode pixel device and complementary triode logic devices for control of the pixel devices. Both the pixel devices and the associated control circuitry are fabricated and interconnected in the same continuous manufacturing process to economically produce full color flat panel display products. Both pixel and logic devices are operated in a gate controlled avalanche mode. Pixels are formed of inorganic or organic EL material ohmically contacted by low work function metal. Luminescence is directly viewed through a glass substrate from the brighter, lateral EL emission which is an improvement over other previously discussed prior art devices. Operating potentials required are those of integrated circuits and are, therefore, low. Power consumption is reduced and the devices present no electromagnetic hazard to a user. The ohmic contacts to EL material and the gate terminal increase operating lifetime and failure problems of other previously discussed prior art direct current (DC) operated devices.
Although superior to other prior art devices, the pixel element devices and flat screen displays comprising these devices disclosed in U.S. Pat. No. 5,656,883 exhibit certain shortcomings and limitations. The deposition and subsequent etching process used to manufacture these devices is relatively expensive. Power requirements, although low when compared with other prior art devices, can still be substantial. The beneficial effects of gate control on power consumption leave room for economic improvement. The pixel structures and flat panel displays made therefrom are rigid and limited in geometry by the shape of the rigid substrate. Furthermore, the devices are somewhat less than robust in certain environments.
In view of the prior art, and object of the present invention is to provide a pixel element using conjugated polymer electroluminescent (CPEL) material which is less costly to manufacture in that the CPEL material can be spun on the substrate and therefore does not require expensive deposition systems used in prior art devices. These materials are also referred to as light emitting polymers (LEPs).
Another object of the present invention is to provide pixel devices which are higher in efficiency and require lower power to operate that prior art devices.
Yet another object of the present invention is to provide pixel elements that can be used to form flexible display panels rather that the rigid display panels taught in the prior art.
Still another object of the invention is to provide a pixel element with improved and more efficient gate control thereby further reducing power consumption.
Another object of the invention is to provide a flexible color display panel with numerous novel applications.
Yet another object of the present invention is to provide a more efficient optical interface between the CPEL and substrate directly by eliminating the ITO layer of the prior art.
There are other applications of the invention which will become apparent if the following disclosure.